I don’t know if you really believe what you’re saying, but I’ll continue answering anyways. I worked at Manulife, the largest private insurance company in Canada, and ignoring the fact our security team was mostly focused on pen testing (which as you know, in contrast to audits tells you nothing about whether a system is secure), but the audits were infrequent and limited in scope. Most corporations don’t even do audits (and hire the cheapest engineers to do the job), and as a consumer, there’s no way to easily tell which audits covered the security aspects you care about.

If you want to talk about the security of open source more, besides what is already mentioned above, not only are Google, Canonical and RedHat growing their open source security teams (combined employing close to 1,000 people whose job is to audit and patch popular open source apps), but also open source projects can likewise pay for audits themselves (See Mullvad or Monero as examples).

I will concede that it is possible for proprietary software to be secure. But in practice, it’s simply not, and too hard to tell. It’s certainly not secure when compared to similar open source offerings.